1. Field of the Invention
The present invention relates to an engine control and more particularly to the transient-state control of a hybrid drive system for vehicles, in order to reduce emissions in a hybrid vehicle comprising at least one electric machine and at least one thermal engine for driving the vehicle.
2. Description of the Prior Art
Accounting for emissions in transient state (generated by the change from one steady-state operating point to another steady-state operating point) from an internal-combustion engine is a very important objective. Indeed, emissions from current engines are highly sensitive to operating deviations. Furthermore, future type approval driving cycles will impose more transient phases. In the case of a hybrid vehicle equipped with a diesel engine, the problem is twofold. First, a compromise has to be reached between the NOx emissions (nitrogen oxides, pollutants emitted by the thermal engine) and the fuel consumption for the steady-state operating point, and second, the transient operating phases of the engine, during which NOx emission peaks are observed, have to be managed.
An example illustrating the transient-state emissions problem is given in FIGS. 1a) to 1d). This example shows the first acceleration of the extra-urban part of an NEDC type driving cycle (New European Driving Cycle, which is a motor vehicle drive cycle intended to mimic in a reproducible manner the conditions encountered on European roads, mainly used to measure vehicle consumption and emissions) for a hybrid drive system equipped with an exhaust gas recirculation system (EGR). During gear ratio change phases, injection cut-off necessarily leads to a decrease in the richness at the exhaust. Burnt gas availability at the intake is therefore limited when injection is restored upon acceleration. In this case, convergence of the burnt gas fraction at the intake to its setpoint is not instantaneous. Its response time depends on the burnt gas transport time in the EGR circuit. During transient phases, the composition of the gases at the intake (and therefore in the cylinder) does not converge to its setpoint and the NOx emissions exceed the static-state emissions. The amplitude of these emission peaks is high in relation to the expected levels under steady conditions and these peaks represent, according to the type of driving cycle, a not insignificant part of the cumulative emissions.
Over a European driving cycle (NEDC), the transient part does not exceed 15% of the total NOx emissions. On the other hand, for an American driving cycle of FTP type (Federal Test Procedure, a drive cycle used in the USA to measure emissions), the transient part represents more than 40% of the total emissions, as can be seen in Table 1. This table shows a comparison between the measured NOx emissions and the quasi-static NOx emissions for two driving cycles (NEDC and FTP) and the transient part of the total emissions for each cycle.
TABLE 1DRIVING CYCLENEDCFTPExper-TransientExper-TransientimentStaticpartimentStaticpartNOx [mg/km]1099015%17010042%Consumption4.54.3 3%4.74.4 6%[l/100 km]
Accounting for the transient part therefore is an important issue regarding engine control. In this context, thermal engine hybridization represents an option with a high potential. The objective is to limit of thermal engine stress under transient conditions using the electric machine or an alternator starter for compensating for the thermal engine torque setpoint.
Energy supervisors have been developed to control such a hybrid drive system. There are several methods based on optimal control. This type of energy supervisor is presented in the following documents:    A. Sciarretta, M. Back & L. Guzzella, “Optimal Control of Parallel Hybrid Electric Vehicles”, IEEE Transactions on Control Systems Technology, vol. 12, no. 3, May 2004.    A. Chasse, A. Sciarretta & J. Chauvin, “Online Optimal Control of a Parallel Hybrid with Costate Adaptation Rule”, Proceedings of the IFAC Symposium on Advances in Automotive Control, Munich, Germany, Jul. 12-14, 2010.    O. Grondin, L. Thibault, Ph. Moulin, A. Chasse & A. Sciarretta, “Energy Management Strategy for Diesel Hybrid Electric Vehicle”, Proceedings of the 7th IEEE Vehicle Power and Propulsion Conference, Chicago, USA, 6-9 Sep. 2011.
However, although these energy supervisors make it possible to manage the static states of the hybrid drive system, they are not suited for the management of transient states.
The thermal engine is considered to be a purely quasi-static system. This hypothesis is acceptable if the engine torque is considered but it is wrong if the emissions are considered. Thus, the part of the emissions produced during transient operation phases in relation to the total emissions is not insignificant. It is therefore necessary to account for the transition between two steady points selected by the static supervisor.
FIGS. 2a) and 2b) show the impact of the torque gradient on the NOx emissions. In order to manage the transient operation phases of the engine, heuristic methods intended to limit the value of the torque gradient required from the thermal engine, while using the electric machine to provide the missing torque and to meet the wheel setpoint, have been developed. This type of heuristic method is illustrated in the following documents:    N. Lindenkamp, C.-P. Stöber-Schmidt & P. Eilts, “Strategies for Reducing NOx and Particulate Matter Emissions in Diesel Hybrid Electric Vehicles”, SAE Paper no 2009-01-1305, 2009,    O. Predelli, F. Bunar, J. Manns, R. Buchwald and A. Sommer, “Laying Out Diesel-Engine Control Systems in passenger-Car Hybrid Drives”, Proceedings of the IAV conference on Hybrid Vehicle and Energy Management, pp. 131-151, Feb. 14-15, 2007.
However, this type of empirical strategy does not allow finding the optimum trajectory and adjustment of the gradient slope depends on the engine, its calibration and the drive mode. In fact, the limits of these gradients have to be calibrated for each cycle.